The lineage of feathered dinosaurs that gave rise to birds was able to survive while all other dinosaurs went extinct because they shrank over evolutionary time, according to a new study published Tuesday.
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An international team of researchers found that the small body sizes of a group of dinosaurs known as maniraptorans was the key to their evolutionary success and also a contributing factor to the diversity of bird species known today.
"Dinosaurs aren't extinct; there are about 10,000 species alive today in the form of birds. We wanted to understand the evolutionary links between this exceptional living group, and their Mesozoic relatives, including well-known extinct species like T. rex, Triceratops, and Stegosaurus," says Roger Benson, a professor of earth sciences at Oxford University and a co-author of the study.
'The Maniraptorans were able to maintain rapid rates of body size evolution. They experimented a lot with different body sizes and ecological niches, which allowed new adaptations to arise very quickly.'—David Evans, Royal Ontario Museum paleontologist
Maniraptorans had many bird-like features, including feathers, large brains relative to body size compared with other dinosaurs and a highly variable diet, says David Evans, a paleontologist at the Royal Ontario Museum and one of the study's leaders.
But it was a series of rapid adaptations and physical modifications leading to smaller species that kept the maniraptorans alive while the other dinosaurs floundered over evolutionary time.
Evans and his colleagues from multiple global institutions compiled a data set of body masses for over 400 dinosaur species — more than half of all the dinosaurs that have been named — using the thickness of femur bones to estimate total body mass.
The variation in body size of dinosaurs is one of the great mysteries of evolutionary biology, Evans says, with the biggest dinosaur in the data weighing 90 tonnes while the smallest was just 15 grams.
After analyzing the database, the largest of its kind ever assembled, the team found that after dinosaurs evolved around 220 million years ago, many species underwent a period of quick evolutionary changes that allowed them to take advantage of different ecological niches and establish their dominance in the early Triassic landscape.
But after the “initial burst,” Evans says, almost all lineages slowed down. The notable exception was the maniraptorans.
“The maniraptorans were able to maintain rapid rates of body size evolution. They experimented a lot with different body sizes and ecological niches, which allowed new adaptations to arise very quickly. They seemed to have a very high level of evolvability,” says Evans.
Body mass is so critical to species’ survival because it determines many aspects of an organism’s ecology and lifestyle. Metabolic rate, food consumption, energy use and the amount of physical space a species needs are all dependent on body mass, Evans says.
Researchers will now try to determine exactly what characteristics of the maniraptorans facilitated their high evolvability, while other dinosaurs seemed unable to adapt to the changing environment that eventually led to their extinction.